The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
CVS contact verification signal
DA destination address
E-UTRAN evolved UMTS terrestrial radio access network
FCC Federal Communications Commission
IBSS independent basic service set
ID identifier
IEEE Institute for Electrical and Electronics Engineers
MAC medium access control
RX receive
SA sender address
SSID service set identifier
TVBD television band device
TVWS television white spaces
TX transmit
WLAN wireless local area network
To alleviate congestion in conventional cellular spectrum, research has turned recently to exploiting license-exempt radio spectrum such as the industrial, scientific and medical (ISM) band and what in the United States are known as TVWS, which is a particular portion of the license-exempt spectrum which was at one time set aside for television broadcast but which in recent years has become increasingly available for other radio communications. In general license-exempt radio spectrum is sometimes referred to as a shared band, contrasted with conventional cellular systems which utilize radio spectrum for which the system operator holds a license from a government regulator such as the FCC in the United States. Administration of wireless operations on such license-exempt bands is quite country-specific at least at this early stage, with the FCC implementing regulations for use of the TVWS in the United States.
In the United States it is envisioned that there will be whitespace or TV band databases indicating, for specific geographic areas or geo-locations, which portions of that license-exempt spectrum are available to parties other than those involved with television broadcasting. Such portions may be identified in the relevant database as indexed channels, bandwidth and center frequency, upper and lower frequency bounds, or other frequency-definitive parameters.
Relevant to TVWS in the United States, the FCC defines two concepts for aiding users in finding available channels; a TV bands database as summarized above and the geo-location capability. See for example document FCC 10-174; SECOND MEMORANDUM OPINION AND ORDER; UNLICENSED OPERATION IN THE TV BROADCAST BANDS—ADDITIONAL SPECTRUM FOR UNLICENSED DEVICES BELOW 900 MHZ AND IN THE 3 GHZ BAND (adopted and released Sep. 23, 2010). The TV band database is to maintain records of all authorized services in the TV frequency bands and so is capable of determining the available channels as a specific geographic location. Such available (or equivalently the used) channels are provided as lists to TVBSs that have been certified under the FCC's equipment authorization procedures. Some of the TVBDs will have a geo-location capability, and those devices should be able to determine their own geographic coordinates within a certain level of accuracy (+/−50 m). This capability is used with a TV bands database to determine the availability of TV channels at a TVBD's geo-location. The FCC defines several types of TVBDs as follows based on those devices' characteristics.
A fixed TVBD is located at a specified fixed location and has the following functions/capabilities: it can select a channel from the TV bands database; it can initiate and operate a network (by sending enabling signals to other fixed TVBDs and/or personal/portable TVBDs); and it can provide to a Mode I personal/portable device (see below) a list of available channels on which the Mode I device may operate (currently, above TV channel 20) and a supplemental list of available channels for Mode I devices (these available channels are adjacent to occupied TV channels and are those on which a fixed TVBD cannot operate). Examples of what may operate as a fixed TVBD in the LTE system include an access node/eNodeB, a mobility management entity MME, a serving gateway S-GW, a local gateway L-GW, and a packet gateway P-GW. Similarly functioning nodes in other radio access networks may also serve the functions of a fixed TVBD. The maximum power a fixed TVBD is allowed to deliver to its TX antenna shall not exceed 1 W, and the maximum power spectral density (any 100 kHz during any time interval of continuous transmission) is 12.2 dBm.
A Mode I personal/portable device is another of the FCC's TVBD types. This type does not use any internal geo-location capability it may have (if any) to find its TVWS channels so even if it can access a TV bands database the mode I device must obtain its channel list from either a fixed TVBD or from a Mode II personal/portable TVBD (see below). A Mode I device may operate only as a client or dependent station/device, but not as an enabling station/device.
A Mode II personal/portable device is a portable device having similar functions as a fixed TVBD, but does not need to transmit/receive signals at a specified and fixed place. For personal/portable TVBDs, the maximum effective isotropic radiated power (EIRP) is 100 mW (20 dBm). If the personal/portable TVBD does not meet the adjacent channel separation requirements (the distance between the TVBD and the TV station is smaller than the minimum distance requirement), the maximum EIRP is set to 40 mW (16 dBm). The maximum power spectral densities for personal/portable devices operating adjacent to occupied TV channels is −1.6 dBm, otherwise 2.2 dBm.
And finally the FCC has designated a sensing only device, which is a personal/portable TVBD that uses spectrum sensing to determine a list of available channels. It can use the frequency bands 512-608 MHz (TV channels 21-36) and 614-698 MHz (TV channels 38-51). Currently, the FCC defines spectrum sensing only for personal/portable TVBDs. The maximum power spectral density for sensing only devices is −0.8 dBm.
The IEEE 802.11 af standard being drafted (D1.02; June 2011) is intended to amend the 802.11 specification for TVWS operation by, among other aspects, fulfilling the above requirements. The document FCC 10-174 cited above further requires that Mode I devices are to be enabled by a (fixed or) Mode II device (called enabling station), and then once enabled the Mode I device may start transmission on an available television channel or channels (more generally, a TVWS frequency resource).
A problem arises in the procedure for enabling a Mode I device. Having first obtained the list of available TVWS channels, the Mode I device must receive a CVS at least every 60 seconds to verify its list. Such as CVS signal is shown at reference number 412 of FIG. 4 in the above-referenced co-owned U.S. patent application Ser. No. 13/184,702. At §15.71 of the above referenced document FCC 10-174 the CVS is defined as follows:                “(b) Contact verification signal. An encoded signal broadcast by a fixed or Mode II device for reception by Mode I devices to which the fixed or Mode II device has provided a list of available channels for operation. Such signal is for the purpose of establishing that the Mode I device is still within the reception range of the fixed or Mode II device for purposes of validating the list of available channels used by the Mode I device and shall be encoded to ensure that the signal originates from the device that provided the list of available channels. A Mode I device may respond only to a contact verification signal from the fixed or Mode II device that provided the list of available channels on which it operates. A fixed or Mode II device shall provide the information needed by a Mode I device to decode the contact verification signal at the same time it provides the list of available channels.”        
In practice the transmission interval of the CVS signal should be more frequent, because in case there is only one CVS transmission during a 60 second period a mode I device which misses it would have to stop operation on TVWS and get re-enabled before it can transmit again. Or if the 60 second time limit is approaching the Mode I can request CVS enablement from the Mode II device which originally provided the list, in which case the Mode II device does not have to track the 60 second timer for every device which it has enabled.
The FCC regulations provide that the Mode II device which provided the channel list for a Mode I device is also required to transmit CVS signaling to each of its enabled devices. But a given Mode II device may enable several Mode I devices on several different channels, and herein lies the problem which is illustrated by example at FIG. 1.
There is a Mode II AP 24 which enables communication on several channels, shown by example as starting from channel #20 (Ch20) up to channel #N (Ch#N). The enablement may be non-contiguous meaning that certain channels between Ch20 and Ch#N will not be available at this particular location. By enabling multiple STAs 20, 22, 23 and potentially also APs (mode I APs) 21 on different channels, on different parts of spectrum, the enablement signaling load of Mode II AP increases. For multiple Mode I devices enabled by a single Mode II device, this is too high of a burden to the enabling device 24 which must frequently re-check the TVWS database and redistribute the channel list. The conventional procedure is also seen to be too high a burden on the enabled Mode I devices 20-23.
Apart from the documents FCC 10-174 and IEEE 802.11af noted above, also relevant to these teachings are:                document IEEE 802.11-11/908r1 entitled “Secure Enablement and CVS without Persistent Association”, by Qualcomm (Jul. 5, 2011);        Section 8.3 of IEEE Draft P802.11-REVmb™/D8.01 (May 2011) concerning frame type formats; and        3GPP TS 36.331 V10.2.0 (2011-06) (Release 10).        